BMW M20 Engine Overhaul, Rebuild, Restoration, and 2.7 Stroker

This BMW M20 engine was offered to me and the deal was too good to pass up. Supposedly it was taken from a parts car with 88k miles. It's from a 1988 325is. I know it's an 88 because it has the later style coolant system with the metal coolant pipe across the front. And it's an "is" because it has a oil filter housing set up for an oil cooler. I got lucky!

Or did I? As I dismantle it I'm finding someone has been through this engine quite a lot. Nearly every bolt I touch is loose already. The timing cover is held on with one bolt, there are hoses missing, and the exhaust manifolds are already removed. And the kicker - the cylinder head is new/fresh. There's not a smudge anywhere inside and half the rocker arms are new! But I can't be so lucky - the timing marks between the cam sprocket and the crank pulley are off. It's off by 1.5 teeth on the cam mark. Since the M20 is an interference engine this can be BIG trouble. I'm hopeful that someone just reassembled the engine hastily and left it off a tiny bit.

Update: after removing the cylinder head and inspecting it and the pistons, I think I have the whole story on what happened. As is too common on the E30 325, the timing belt probably broke, which sent the valves into the pistons. There are grooves in the exhaust valve reliefs on the pistons that shouldn't be there (the intake side looks good). So the head was pulled and rebuilt but was not milled. Then the pistons were cleaned up but not removed from the block. And the motor was reassembled with all new gaskets and driven for a little while. Then the car was sold. The last owner of the engine started taking it apart but took a different direction. That's why so many parts are removed or loose. I got very very lucky with this deal!

oil pump gear seen through the hole where the original distributor shaft was located on the first gen M20. the cover over this just pulls straight out. there is a rubber seal on the cover that could be a source for oil leaks.

intermediate shaft gear and oil pump gear. the rod from the oil pump goes in here.

needle bearings for the oil pump gear

cover for the distributor shaft hole. note the needle bearings here too and the rubber seal around the cover. it was recommended that I remove this cover before the engine block is thermal cleaned and it sounds like a good idea to replace the bearings and seal anyway (Real OEM diagram: http://realoem.com/bmw/showparts.do?model=1113&mospid=47309&btnr=11_0119&hg=11&fg=10). the cover itself is no longer available from BMW.

cover for the distributor shaft hole. the cover part is no longer available from BMW (but the seal and bearings are)

cover for the distributor shaft hole and the oil pump gear. the gear is actually upside down in this photo - the hole end should be exposed.

maybe the dumbest thing I have seen so far. flat head screws? these are 30 years old and not coming out easily. these screws hold a plate to the block that keeps the intermediate shaft from spinning out of the block. a regular screwdriver didn't give me enough leverage but a flathead socket in a 3/8 ratchet made it happen.

bare block ready for machine shop.

piston comparison.

piston comparison.

piston comparison.

new bearings for the oil pump drive and intermediate shaft, and block freeze plugs

yes, cleaning the engine stand at 10:30 at night. But a clean stand kicks off the rebuild right.

M20 headers cleaned and almost ready for paint.

M20 engine wiring harness laid out for cleaning

M20 intermediate shaft bearings installed.

M20 crankshaft bores.

ahh home.

main bearings all lubed up. note: oil hole to oil hole.

main bearings all lubed up

this neat little tool is used to push the ring into the cylinder and keep it square to the cylinder walls.

checking ring end gap

pistons and rods. Seems straightforward but one of the toughest jobs so far. I read a lot about the process and one thing rang true: you start out as a newb but finish as a pro. The really tough part is the wire locks holding the wrist pin in the piston. The first two pistons took about an hour each. Cyl 5 and 6 took 13 minutes apiece. The rings are also a pain, particularly the oil rails that sandwich the main oil ring. They get mixed up and block the oil ring from seating in the groove. The trick is to seat the oil ring and then do the oil rails one at a time. The top and 2nd rings are easy.

pistons and rods

broken ring from a poorly designed ring compressor tool. 5 of the other 6 pistons went perfectly fine. Wiseco sells ring sets per piston though so this is just a minor setback.

we have a rotating assembly

the oil is Marvel Mystery Oil

oil pump installed

front crankshaft cover and seals. make sure you put the lock plate for the intermediate shaft on the right way. it will line up on the wrong holes and interfere with the cover. getting the seals over the crankshaft and the intermediate shaft without folding under was tough.

fully assembled front end. this was just put together with the original head bolts, timing belt, and tensioner so I could check valve to piston contact.

M20 throttle body heater delete. I followed the how-to on garagistic.com http://garagistic.com/index.php?page=m20-thermostat-housing---throttle-body-heater-delete). It was easier than I thought - I punched the complete tube out instead of cutting and then punching. I drilled out the hole and then tapped it with the 1/4-18 NPT tap. all of that is straightforward. the countersunk plug threads in fine but gets very tight too soon. I thought it would thread in all the way but nope. I've cleaned the threads as best I can but both plugs I bought go this far and no more. probably not a big deal...

M20 oil level sensor block-off plate

my head bolt saga. set #1 (Victor-Reinz): 1 very bent bolt, a few slightly bent bolts. Yes, a bent head bolt. set #2 (Genuine BMW): waited 3 days for them to come in. Threaded fine and no bent bolts. But I over-torqued them in the head! It was a dumb mistake. set #3 (Victor-Reinz): 1 very bent bolt, a few slightly bent bolts. Yes, more bent bolts from V-R. From this point it will only be BMW bolts even if I have to wait another three days and get a new BMW head gasket (no V-R gaskets either!). I'm losing steam because nothing else can be done without bolting down the head. At least I can work some more on pulling the 323i motor.

partially assembled. the head is not bolted down so there's no timing belt yet. this is sort of a dry run to see what I'm missing.

bitch tube spring. I've seen a lot of ways to compress the spring but mine has this clip. booya!

front end done minus the cap and rotor and the wiring harness holder (I hate waiting for parts). the only new parts in this picture are the water pump, crankshaft pulley, and the alternator bracket.